Unfortunately, Apple has never provided a public API to programmatically check if Mission Control is active.

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It turns out that there is nevertheless an easy way to check if Mission Control is active (code snippet below): We just ask which windows are present on the screen. If there is a specific window present owned by the dock process and with a vertical origin of -1.0, this will be a “Dock Window” as shown by Mission Control only […] Of course this is slightly hackish and might break in the future […]